1. Field of the Invention
The present invention relates to a surface-mounting type of electronic circuit unit and a method for fabricating the unit.
2. Description of the Related Art
Typically, components in surface-mounting type electronic circuit units are chip components such as resistors and capacitors or semiconductor components such as transistors that are soldered on solder lands in a conductive pattern provided on a rectangular, platelike substrate. These circuit components are covered by a shield cover. Edge electrodes are provided on side surfaces of the substrate, and respective conductive patterns provided on both top and back surfaces of the substrate are conducted to each other by the edge electrodes. The edge electrodes are formed by printing a silver (Ag) paste such that they extend over the both upper and lower conductive patterns derived on ends of the substrate, curing the paste, and plating the paste with nickel (Ni) and gold (Au) layers. Thus, when the electronic circuit unit is surface-mounted on a base substrate, the edge electrodes can be connected on lands of the base substrate using a conductive adhesive.
Recently, the circuit components have been miniaturized. For example, an ultracompact chip resister and chip capacitor having outside dimensions of about 0.6*0.3 mm have been put into practical use. In the conventional electronic circuit unit in which such ultracompact circuit components are used, and in which the pitch between the circuit components mounted on the substrate is correspondingly narrowed, the electronic circuit unit can be miniaturized only to a certain extent, i.e. miniaturization of the circuit components is limited. Moreover, when a number of circuit components are mounted on the substrate, a soldered portion of each circuit component may not be shortened during mounting. Thus, the narrowing of the pitch among the components is also limited. These limitations have been a major factor impeding further miniaturization of the electronic circuit unit.
In addition, in the conventional electronic circuit unit, although the unit is configured such that the silver paste is printed on the side surfaces of the substrate to provide an electrical connection between the upper and lower conductive patterns, it is difficult to control print dripping of the silver paste. Further, the silver is cracked after curing when the treatment temperature is not appropriately controlled. Due to both of these problems, the process for forming the edge electrodes is extremely complex and difficult, as well as being relatively time consuming and expensive.
The invention, which was made in view of the problems inherent in the conventional mounting technique, provides a surface-mounting type of electronic circuit unit that can simplify the process for forming the edge electrodes and is suitable for miniaturization.